Field of the Invention
The invention relates to an actuator arrangement comprising a first actuator for driving a driven element in a first direction and a second actuator for driving the driven element in a second direction opposite to the first direction, wherein the first and second actuators are configured to work in a counter-acting manner for moving the driven element in said first and second directions. Further, the invention relates to a control surface arrangement, especially for an aircraft, comprising a control surface element and an actuator arrangement for a controlled movement of the control surface element.
Background Information
U.S. Pat. No. 5,224,862 discloses an actuator arrangement for driving a control surface of an aircraft in the form of a flap arranged at a blade of a helicopter. Especially, U.S. Pat. No. 5,224,862 describes a piezoelectric helicopter blade actuator, wherein electrically deformable material such as a piezoelectric material is used to deform a deflectable flap on an airfoil such as a helicopter blade. The electrically deformable material is controlled to deflect the flap in a manner to control vibrations transmitted from a helicopter blade to the helicopter airframe.
US 2010/0181415 A1 describes a rotor blade for a rotary wing aircraft equipped with a movable rotor blade flap and an actuator arrangement wherein actuators are dynamically connected to a reversibly bendable supporting member for moving the flap.
A further helicopter rotor blade flap and an actuator arrangement therefore is disclosed in U.S. Pat. No. 6,231,013 B1. U.S. Pat. No. 6,231,013 B1 discloses a rotor blade for a helicopter which includes an airfoil body and a servo-actuated flap tiltably connected to the airfoil body. At least one piezoelectric actuator is arranged inside the airfoil member, and connected to the servo-flap via a transmission mechanism and connecting bar linkage so as to tiltingly deflect the servo-flap as needed. Preferably, two counter-acting piezoelectric actuators are connected to the flap via respective transmission mechanisms and connecting bar linkages. An elongation of the piezoelectric actuators in the span width direction of the airfoil member is converted or redirected into an actuating motion in the chord length direction of the airfoil member by the transmission mechanisms. Orienting the actuators to have an elongation motion in the span width direction prevents the high centrifugal accelerations arising in the rotor blade from having negative influences on the operation of the actuators.
There are several possible types of actuators for driving control surfaces such as flaps in helicopter blades. One interesting type are ceramic piezo-actuators which are made from ceramic. Ceramic can withstand high compressive forces but low tensile forces. Hence, it is advantageous to have a compression pre-load on the piezoelectric actuators.
Examples for actuator arrangements using pre-compressed actuators can be found in U.S. Pat. No. 6,294,859 B1. However, when providing pre-compression loads it is necessary to counteract such pre-loads. This brings high loads on hinges or a driven element to be driven by the actuator.
One possible solution to counteract the pre-compression loads is to have one actuator working against a spring providing the pre-compression load. The actuator is put inside a box that is not rigid and the actuator is working against the box. However, this solution leads to losses induced by the counteracting box.
A technology as presently used internally by Eurocopter and shown and explained below with reference to FIGS. 1, 3, and 7 uses an actuator arrangement having a first actuator and a second actuator wherein the first and the second actuators are working against each other. Hence, losses induced by working against flexible boxes or the like are reduced or avoided.